1. Field of the Invention
The present invention relates generally to security papers and, more particularly, to a laminated security paper for use in making banknotes, passports, stock certificates, checks and the like.
2. Description of Related Art
A great number of printed documents require highly reliable means of ensuring their authenticity due to some intrinsic value associated with them. These documents range from banknotes to property titles, and include such things as negotiable instruments, stock certificates, checks, and passports. To be truly of value to the user or holder of the document, the means employed to indicate authenticity must be readily and easily detectible. Moreover, to serve the more general purpose of allowing the public at large to rely on the authenticity of the documents, these means should be permanent, durable, and difficult to replicate. This latter quality is particularly important to preclude, or at least to dissuade, attempts at counterfeiting the documents in order to ensure a maximum degree of confidence in the original document. In the case of banknotes, passports,, checks, and other intrinsically valuable documents, confidence in the authenticity of the document is especially important, as any member of the public might become a holder or user of the document at any time. Thus, creators of such intrinsically valuable documents are most often concerned with providing a means by which the public may verify the authenticity of the documents because, ultimately, it is the unsuspecting public who is most at risk of being duped by counterfeits.
Historically, attempts at guaranteeing the authenticity of intrinsically valuable documents have concentrated on two aspects of the documents: (1) the paper employed as the basis for the document, typically referred to in the art as "security paper," and (2) some security feature either incorporated in the paper or added during the transformation of the paper into the final document. Due to their important role in providing lasting documents and a dependable basis for public confidence in the authenticity of such documents, both the security paper and the security feature must respond to numerous, stringent criteria relating to their design and utility. Criteria for security papers include, both physical and optical requirements. The requirements of security features relate not only to their physical properties, but also to the relative difficulty of their replication and to their reliability in guaranteeing the authenticity of the final document.
The physical requirements imposed on security papers, particularly papers used in the manufacture of banknotes, are generally much more strict than for other types of paper. One such requirement is the "basis weight" or the "grammage" of the paper, usually expressed in grams per square meter, or pounds per ream. For most security papers, the basis weight must fall within a fairly narrow range so as to lend sufficient body, or feel, to the document to satisfy the expectations of holders, while allowing the document to be easily folded and unfolded. Closely related to the basis weight is the "caliper," or thickness of the paper, expressed in fractions of a millimeter or inch. It is important that the caliper of the security papers used in any particular application function well with machines, such as automatic teller machines and high speed sorters, which will inevitably handle the final documents.
Strength and durability are crucial properties of security papers because they give the final document its ability to withstand the use, and abuse, likely during its service life. The tensile strength, expressed in units of force per unit width, represents the greatest longitudinal stress a piece of paper can bear without tearing apart. Other strength parameters include wet tensile strength, breaking strength, tearing strength, and folding endurance. Because banknotes are inevitably folded and unfolded many times during their circulation life, high strength and great folding endurance are required of the papers from which they are made. Banknote paper is typically designed to allow some 5,000 to 8,000 folds during its expected circulation life. It should be noted that ratings of security papers based upon the number of folds the papers can endure without tearing may vary widely depending upon the test procedures employed. Moreover, while permitting a considerable number of folds, modern banknote paper must retain some degree of rigidity, or resistance to bending, sometimes referred to as "crispness," to pass successfully through automatic teller machines and high speed sorters.
Other important physical properties of security papers include porosity, printability, and dimensional stability. Because porosity is related to the soilability of a paper, that is, the tendency to absorb or retain impurities, a relatively low porosity is generally desirable for security papers, to increase the likelihood that the document will maintain a clean appearance throughout its useful life. At the same time, security papers require good printability characteristics to ensure accurate and precise impression in the printing processes employed in the manufacture of the final document, as well as proper penetration and adhesion of inks used in the document production. Printing processes used to make certain intrinsically valuable documents may place extreme demands on the paper. Intaglio, or gravure printing, for example, often used in printing banknotes and other documents, involves tremendous pressures between the printing plates and the paper. The paper selected must withstand these processes without loss of physical integrity. Finally, the paper's dimensional stability is important, both during the manufacturing processes and during the service life of the final document. In particular, the paper should not swell or contract excessively due to changes in humidity that typically occur during the manufacturing processes and during ordinary use.
Security papers typically exhibit certain optical properties that affect their usefulness and acceptability to the public. The most important of these properties are the surface characteristics of the paper, and the paper's opacity. Surface characteristics include color, brightness, gloss, finish and smoothness. These parameters are usually specified by the designer of the final document based upon the aesthetics desired in the document; and various processes, such as coating and calendering, may be used to obtain the desired appearance and feel. The opacity of the paper refers to the relative inability of light to pass through the paper. Opacity is generally represented in terms of the percentage of the light incident on one side of the paper that is not transmitted through the paper to an observer on the other side, but is reflected or absorbed by the paper. High opacity, that is, in excess of about 75%, is generally desirable for security papers. However, variations in opacity may actually serve as security features in the final document, as in the case of watermarks, wherein the image or mark, visible in transmitted light, is formed by causing the paper to take on different levels of opacity in such a way as to define a recognizable mark.
The selection and specification of the security feature to be incorporated into an intrinsically valuable document is often intimately related to the specification of the security paper in connection with which it is to be used. There are several reasons for this. First, the security feature is often a quality of the paper itself, such as a watermark or a particular paper composition. Even where the feature may be said to constitute an element separate from the paper, such features are often imbedded in the paper during the papermaking process. This is true for a great number of security features, including so called security threads, chemical additives, colored confetti, and thin tapes containing microtext or other indicia. In addition, even surface-applied security features must be compatible with the underlying paper to provide the desired adhesion and durability characteristics to the resulting document. Such surface-applied security features include non-reproducible images, such as line art that produces moire patterns when photocopied, photosensitive coatings, and surface decals. Even intricate line art printed on the face of the document may constitute a security feature by virtue of the difficulty with which it is replicated. In all cases, however, the security feature must function well with the security paper selected.
The criteria for an effective document security feature are relatively easy to formulate. Such features should be difficult to replicate to deter potential counterfeiters, or at least to render the fruit of their enterprise less passable to the public. The features should permit ready detection by means available to ordinary holders or users of the final document. For banknotes and other documents on whose authenticity the public at large relies, the features should be discernible and verifiable under ordinary light conditions. Finally, in certain cases, it may be desirable that the security feature afford detection of the movement of large quantities of documents, such as banknotes.
While these criteria may appear straightforward, developing a security feature that reliably satisfies them is not an easy task. Historically, the production of an effective security feature for documents relied upon specialized knowledge or ability possessed by only a tiny segment of the public. Thus, for centuries the rare talents of the master engraver and the specialized knowledge of the printer sufficed to prevent or deter most forgery attempts. Later, the combination of specialized knowledge of several disciplines into the same document became the key to preventing counterfeiting by all but the most determined and able teams of forgers. However, modern advances in color photocopying, scanning, and offset printing technologies have made the production of very convincing counterfeits possible even without such specialized knowledge. To be truly effective against forgery, security features must now be not only difficult to replicate due to the specialized knowledge required for their production, or due to a combination of specialized fields into a single document, but must be very difficult or impossible to duplicate on sophisticated photocopying machines.
Finally, to be useful in producing intrinsically valuable, documents, the security paper and the security feature incorporated in or on the paper must together be capable of transformation into a aesthetically pleasing document, at commercial rates and at a reasonable cost. The governmental, quasi-governmental, and corporate entities that generally issue such documents demand no less than that their documents reflect the character of their issuer and inspire the utmost confidence by both their functionality and their appearance. Thus effective security papers and security features should offer a wide degree of freedom in the design and execution of the final document.
A number of approaches have been proposed in the past for providing a security paper with effective security features. May of the proposed security papers incorporate security features during the papermaking process, that is, on the papermaking machine while the web of the security paper is being formed. Such papers make use of security features such as water marks, security threads, security tapes with or without microtext, confetti, and other devices embedded in the paper prior to completion of the papermaking operation. However, such security papers generally cannot provide a security feature covering the entire surface of the security paper due to the need to drain moisture, from the paper during final processing. In addition, security features such as confetti are not generally localized within the security paper, as is required in many applications. Although security features such as security tapes and threads may be localized in the papers, these devices can cause difficulties in printing, sheeting and other process steps due to the increased local thickness in the paper resulting from their presence.
Another approach to incorporating a security feature in security paper is to laminate finished paper sheets to form a composite security paper. One such security paper is described in U.S. Pat. No. 5,161,829. In that security paper, at least two paper sheets are laminated to form a security paper with an authentication indicator incorporated in the laminate. The authentication indicator is printed on the inner surface of at least one of the paper sheets prior to lamination, and is detectible in transmitted light, but not in reflected light. However, printing authentication indicators on one of the paper sheets can lead to problems with "show-through," or bleeding of the inks used to print the authentication indicators, through the paper sheets to the surface of the security paper. Furthermore, where printing is done on a thin inner paper sheet, this would usually involve the use of thin inks, resulting in substantial masking of the security feature by surface priming and making the security feature less visible or not visible at all. Moreover, the use of paper sheets in the laminate does not result in a composite security paper having physical properties beyond those attainable with more conventional single-web papers. In particular, properties related to circulation life, including folding endurance and tearing strength, are in no way enhanced in the resulting laminate.
Another laminated paper product incorporating a security feature is described in Japanese laid-open application no. 322109/88. That product, a card stock formed by laminating two relatively thick paper sheets on either side of a synthetic substrate, incorporates machine-readable marks on the substrate material. The marks, such as universal product codes, are printed in inks that do not permit transmission of light of near infrared wavelengths. Other marks are printed on the substrate in near infrared-transmitting inks. The resulting marks are not distinguishable in visible light, but can be distinguished and read when the cards are passed between a light emitting diode of the proper wavelength and a photoreceptor. However, the cards described are not intended, nor are they suitable for use as security paper, due to the stiffness and low folding endurance inevitably resulting from their physical structure. Moreover, the security feature taught in this document is not readily discernible to a holder under ordinary light conditions. On the contrary, the marks printed on the substrate cannot be discriminated in visible light and are thus of no use to the ordinary holder without special detection means.
The present invention is directed to overcoming or minimizing the drawbacks of the existing techniques set forth above, and to providing a security paper for documents with intrinsic value having security features that can be readily distinguished by the public, but that are not susceptible to replication by counterfeiters. The invention provides a security feature in the form of a color mark, not obtainable by prior art processes or in known security papers. Furthermore, the invention offers a security paper of substantially uniform thickness, facilitating conversion operations such as printing and sheeting, and allowing the paper to perform well in high speed sorters and automatic teller machines. Moreover, the invention provides security features which may be localized where desired. The security feature may be employed alone or in conjunction with known security devices.